This invention relates to electrochemical cells and batteries whose cell packs comprised of plates and separators are less than fully saturated with electrolyte, and a portion of the cell pack makes abutting contact with the cell or battery container, and more particularly to a means of promoting uniform distribution of the electrolyte throughout the cell pack mass.
Lead-acid cells and batteries are known in which the electrolyte is fully absorbed within porous plates and separators, substantially without any free electrolyte being present elsewhere within the cell. "Limited" or "starved" electrolyte cells capable of recombining internally evolved gases further require that residual void volume remains in the plate pore structure and usually in the separator pore structure as well. A cell of this type which employs highly absorbent microfine glass separator material, compressed between the positive and negative porous plates, and in which internal gas recombination is facilitated by a cell configuration sealed at superatmospheric pressure is described in McCelland et al. U.S. Pat. No. 3,862,861, hereby incorporated by reference. In this recombinant, limited electrolyte cell the absorbent separator material may extend beyond the edges of the plates and along the edges of the cell pack to reabsorb any temporarily freed liquid electrolyte as well as providing vibration resistance to the cell assembly and protection against short-circuiting.
Cells and batteries of the above type typically have cell packs spiralled in jelly-roll form, stacked flat in prismatic form, interleaved in accordion style or the like. With the spiral configuration the electrolyte is introduced through the central void of the spiral cell pack and then follows these paths: (1) The electrolyte travels through the separator starting at the center and spirals to the outside of the jelly-roll; (2) the acid passes through the extended separator portions both over the top and along the bottom of the plates and then up and down the separator material interleaved between the plates, finally impregnating the plate pore structure as well; and (3) the electrolyte passes directly through the separator/pasted plate/separator/pasted plate . . . interfaces in a radial direction until the electrolyte has completely penetrated the wound element. The first enumerated path is inhibited by the length of the separator material and therefore the larger the cell the greater the time period required for absorption of electrolyte. The third enumerated path is slow because of the high density of the plates in comparison to the separator. The majority of the filling action takes place by the electrolyte (acid) traveling along the top and bottom surfaces of the element through the compressed separator extensions and then up and down the separator until the electrolyte meets at the middle of the separator web. Very similar paths are followed by the electrolyte in the filling of prismatic batteries except that the electrolyte reaches the bottom of the cell pack other than by going through a central void.
In some instances the above filling techniques are particularly slow and, for larger size cells, may result in nonfilling of middle portions of the cell pack -the so-called "dry band" phenomenon which not only reduces the capacity of the cell but leads to premature failure by dendritic shorting or passivation in the vicinity of the dry band.